In the last decade, there has been an increased interest in the use of orbital space environment to carry out experiments associated with low-gravity solidification processes.
In a low-gravity environment, the levitation of large samples can be achieved by using only small forces to overcome residual spacecraft motion. One can easily levitate and independently heat materials to elevated temperatures using only modest amounts of heating power. Materials can be prepared and studied without introducing crucible effects. Supercooling becomes a natural extension in such an environment since the elimination of crucibles allows ease of superheating to dissolve heterogeneous nucleants and the elimination of container-induced nucleation. Thus, the space environment can allow for large supercooling in "bulk" form for various high-temperature metals, alloys or glasses.
The supercooling of liquid metals and alloys below their equilibrium melting temperature has been widely studied. In normal container casting techniques, only modest supercooling of a few degrees takes place since catalytic sites usually initiate nucleation and crystal growth. Catalysis for heterogeneous nucleation includes contact with crucible walls and various undissolved dispersents within the melt. In order to supercool a liquid metal to the maximum extent, various techniques have been developed to remove container nucleation effects and to eliminate heterogeneous nucleants from the melt. One technique generally known as the emulsion technique involves breaking the metal up into small droplets (diameter 2-10 .mu.m) and dispersing the droplets in a carrier fluid or placing the droplets on clean substrates. The emulsion technique helps eliminate both types of nucleation provided that the droplets are covered by an inert film consisting of an oxide, sulfide or salt. The droplet technique allows major supercooling because it disperses the heterogeneous nucleants to only a few of the drops and thereby frees the remaining drops to supercool to some lower nucleation temperature.
In addition to the previously discussed techniques, another technique developed to provide for containerless supercooling and solidification of small molten drops is commonly known as the droplet technique. Small molten drops of refractory metals are formed by utilizing the discharge of a capacitor or a pulsed laser into fine wires of the sample such as Nb and Ta. The wires melt very rapidly and form small droplets. The droplets are thrown in various directions and at various velocities by the force of the exploding wire. The droplets are solidified as small spheres and supercooling may be provided by cooling in a gaseous or liquid helium environment. The amount of supercooling achieved was believed to be in the range of several hundred Kelvin. This technique provided spherical single crystals of metals with diameters in the range of 0.1 to 0.5 mm. The exploding-wire technique has several disadvantages. This technique is limited to samples in wire form only, which eliminates the processing of glasses and brittle alloys. Also the samples produced are extremely small, in the range of 0.1 to 0.5 mm. Due to the method of heating, this techniques offers very little or no control over heating, melting, and solidification of the samples.
Another technique for studying containerless supercooling in bulk form involves the use of electromagnetic (EM) levitators which operate by passing a high frequency oscillating current through a precisely formed coil. A field is set up in the coil which can levitate and heat a sample. The sample can be heated, melted, cooled, and resolididified by control of the coil current and a flowing cooling gas. Electromagnetic levitators suffer from the disadvantage of a low coupling efficiency which depends upon sample size and shape. This technique also develops large thermal gradients within the sample which result in severe convection currents. The severe convection current results in the loss of low vapor pressure phases within alloy samples and also leads to smaller amounts of supercooling due to possible dynamic nucleation effects and the introduction of surface oxides into the sample interior. Another disadvantage of levitation is the limitation to independently control sample levitation size and temperature, particularly for higher melting point materials. Likewise, EM levitation cannot simulate low-gravity conditions and, thus, they cannot suppress phase separation processes such as sedimentation and solutal convection. Both techniques also are limited to electrically conducting samples.
While several of the above developments and studies have identified the beneficial reasons for using a containerless environment in achieving major supercooling, a direct quantitative comparison between using containerless techniques and in using dispersion or droplet techniques cannot usually be made. The reason for this is that accurate supercooling data is usually not available for freely falling molten particles.
Zero gravity enrivonments have been provided for other purposes such as calibrating instrumentation used in outer space as shown in U.S. Pat. No. 3,408,870.
Accordingly, an important object of the present invention is to provide apparatus to supercool and solidify pure materials and alloys in a containerless, low gravity environment.
Another important object of the present invention is to make certain metastable alloys and compounds in bulk form which have heretofore been unable to be made in such form.
Still another important object of the present invention is to provide apparatus for economically producing spherical single crystals of pure metals and alloys.
Still another object of the invention is to provide an apparatus to produce droplets of oxide glasses or amorphous alloys.